The present invention relates to a circuit for saving power consumption in an electronic appliance during a standby state. More specifically, the present invention relates to a circuit, for use in a home appliance (TV/VCR) consisting of a combination of a TV and a VCR (video cassette recorder), for saving power consumption during a standby state by reducing the output voltage during the standby state to save power consumption and to improve reliability.
FIG. 1 illustrates a conventional power supply circuit of the type used in home electronic appliances. As shown in this drawing, the circuit includes: a power source rectifying section 102 for rectifying an AC voltage supplied from an electrical outlet via a power plug 101; a switching section 103 for applying the DC voltage from the rectifying section 102 to the primary side of a transformer T1 through a switching operation; a secondary rectifying section 104 for rectifying the rectangular waves induced in the secondary side of the transformer T1 to provide the output power for the appliance, such as a monitor and a video apparatus; a reference voltage detection control section 105 for comparing the output voltage of the secondary rectifying section with a reference voltage and providing control signals from an output thereof; and a feedback section 106 for adjusting the feedback amount of the output voltage of the secondary rectifying section 104 under the control of the reference voltage detection control section 105 to control the switching of the switching section 103.
The feedback section 106 controls the on/off time of the switching section 103, so that a stabilized voltage can be obtained. The feedback section 106 is composed of a photo coupler consisting of a photo diode and a photo transistor. When a current is supplied to the photo diode, the photo coupler is turned on in such a manner that a current is supplied to the base of a photo transistor as a result of the light from the photo diode illuminating the base, so that the photo transistor turns on and a current flows from the collector to the emitter of the transistor. Under this condition, the photo diode and the photo transistor are electrically insulated from each other.
In the circuit of FIG. 1 constituted as described above, the AC voltage is rectified and smoothed by the power source rectifying section 102, before being output as a DC voltage to the switching section 103. The switching section 103 is turned on and off under the control of the feedback section 106, so that the output voltage of the power source rectifying section 102 will be induced on the secondary side of the transformer T1. The rectangular waves, which are induced on the secondary side of the transformer T1, are rectified and smoothed by the secondary rectifying section 104 to become a DC voltage which is to be supplied as the power for a monitor and a video apparatus. Further, the output voltage of the secondary rectifying section 104 is supplied to both the reference voltage detection control section 105 and the feedback section 106.
Under this condition, the reference voltage detection control section 105 compares a reference voltage with the output voltage of the secondary rectifying section 104 for maintaining a constant voltage level. If the output voltage of the secondary rectifying section 104 is higher or lower than the reference voltage, a control signal is emitted to the feedback section 106, so that the feedback amount will be adjusted.
The feedback section 106 adjusts the feedback amount of the output voltage of the secondary rectifying section 104 under the control of the reference voltage detection control section 105 for the purpose of controlling the switching on/off time of the switching section 103, thereby obtaining a constant voltage all the time. Accordingly, when the user watches a TV or carries out a programmed recording by utilizing the circuit of FIG. 1, there will be no problem.
However, the voltage in the secondary side of the transformer T1 is also maintained at a high level during standby states, resulting in unnecessary power consumption.
U.S. Pat. No. 4,737,851 discloses a circuit which is entitled "On/Off Control Circuitry for Television." According to this circuit, the generation of a deflection current is prevented during a standby state in a television, thereby saving power consumption. That is, the circuit of U.S. Pat. No. 4,737,851 includes: a deflection circuit for producing a deflection current in a deflection coil during the normal mode; a voltage source for providing a first supply voltage during the standby state and during the normal mode; and a control circuit coupled with the deflection circuit for controlling the operation of the deflection circuit during the normal mode and for preventing the formation of a deflection current during the standby mode. Thus, the operation of the deflection circuit is controlled during the normal mode, while the generation of the deflection current is prevented during the standby mode. In this way, the monitor is turned off, so that power consumption is reduced during the standby state. However, the power consumption for the VCR itself is not saved by the circuit of the latter patent.